Question

So in some of the codes I see, they access an objects ivar directly instead of using accessors . What are the advantages of using them instead of accessors?

So how would this

thing = object->ivar 

differ from this?

thing = object.ivar

Thanks.

Answer 1

First let me say, I totally loathe the Objective-C dot notation. It sacrifices understandability for brevity and that is a bad thing. In fact, the other two answers here both show evidence of the kind of confusion dot notation introduces.

Having got the rant out of the way, I’ll now try to answer the question.

Under the hood, Objective-C objects are implemented as pointers to C structs. This is why

obj->ivar

sometimes works. Given that it’s a C struct

(*obj).ivar

should also work exactly as you would expect for C. Having said that, you can make ivars private or protected, in which case using the above outside a scope where they are visible will cause a compiler error.

The dot operator when applied to an Objective-C object (which is a pointer don’t forget) has a totally different meaning. It’s syntactic sugar for sending an accessor message to the object meaning that:

foo = obj.property;
obj.property = foo;

is identical in effect to

foo = [obj property];
[obj setProperty: foo];

That is all there is to dot notation. If you go through your code changing all instances of the first form to instances of the second form, you have done everything the compiler does wrt dot notation.

In particular

• you do not need a declared @property to use dot notation. You can declare the set and get accessors in the traditional way as Objective C methods, although it is definitely best practice to use @property declarations for things that are logically properties.
• you do not need a backing instance variable. There’s no reason why your getters and setters can’t calculate values.

Given the above, the major difference between obj->ivar and obj.ivar is that the former modifies the ivar directly and latter invokes an accessor, this means that the latter can do any memory management stuff needed (retains, releases, copies etc) and can also invoke key value observing.